Microphone



F. RIEBER Oct. 9, 1934.

` s sheets-sheet 1 Filed Dc. 15, 1950 Lilllhllld F. RIEBER 1,975,801

MICROPHONE Filed npc.- 15, 1930 s sheets-sheet. 2

Oct. 9, 1'934.

F. RIEBER MICROPHONE Filed Dec. l5, 1930 3 Sheets-Sheet 3 WMM l y, arnev Oct. 9, 1934.

Patented a. 9, 1934 I UNITEDl STATES PATENT OFFICE MICROPHONE Frank Bieber, San Francisco, Calif., assignor to I Sound Laboratory Corporation,

Ltd., San

This invention relates to the so-called condenser microphones, in which sound in air acts upon the device to cause corresponding variations in electrostatic capacity, thesevariations in capacity ,5 reacting on an appropriate electric circuit to causevariationsinelectric energy delivered therefrom; and more particularly to a microphone composed of a plurality of independent cells, acted upon by the pressure waves in the air.

Condenser microphones of the general class described have long been known. l.It is customary in constructing such microphones in their most effective form to employ a diaphragm of light material such as aluminum or its alloys, and to stretch the diaphragm tightly in a frame similar to a drum head. This tightly stretched diaphragm is placed at a very short distance from a hat metal plate, the diaphragm and the plate being carefully insulated one from the other only at the points of contact. That is, where the diaphragm is spacedly secured to the at metal plate. Occasionally recesses or slots are cut in the plate for the purpose of. permitting the air contained between the diaphragm and the back plate to enter and leave this space readily during the periods of vibration of the diaphragm.

This tight stretching of the diaphragm is necessary for two reasons. First, it is highly desirable to have an extremely at diaphragm in order that it may be brought in close juxtaposition to the back plate and all points. This stretching removes any wrinkles or irregularities in the diaphragm and permits this close juxtaposed relationship.

Second; it is desirable to maintain the free vibratory frequency of the diaphragm at an extremely high pitch, considerably above that of any sound which it is desired to receive with the device. If this precaution is not observed, sounds 40 of the particular frequency to which the diaphragm is resonant will be received and conveyed by the device far more effectively' than sounds of other frequency, thus producing distortion.

The careful insulation of the back plate from the diaphragm is made necessary by reason of the fact that the device must be operated at extremely high voltages, and further, on account of extremelv high working impedance of the micro-v phone at ordinary frequencies. Any leakage of current across the insulation would detract very.

appreciably from the efficiency of the device and would further, on account of the irregularity yof current flow commonly accompanying leakage current, cause the device to-become noisy.

It is therefore an object of this invention to provide a device of the character described in which the insulation between the metal plate and diaphragm is exceptionally perfect and stable and thereby prevents the possibility of short-circuiting due to the contact of the diaphragm with the back plate.

It is a further object of this invention to provide a device in which the diaphragm may be made of far thinner and lighter material than that heretofore in use. This thinning is accomplished without any corresponding structural disadvantages in the diaphragm, by subdividing the diaphragm into a plurality of mutually independent cells.

It is a further object of this invention to provide a ydevicein which the effective diaphragm surface may be increased to embrace area much greater than those of the diaphragm at present in use. This likewise -is accomplished by subdividing the diaphragm into a plurality of mutually independent cells, which in effect forms with the metallic back plate a plurality of mutually independent microphones, each functioning independently of the other.

It is a further object of this invention to provide a device in which the inherent vibratory frequency of the diaphragm member and the effective damping applied to the diaphragm member may both be maintained at an extremely advantageous value.

It is a. further object of this invention to provide a device which will be relatively inexpensive to manufacture, positive in action, rugged in construction, and-insensitive to changes in atmospheric moistures.

My invention possesses many other advantages, and has other objects which may be made more easily apparent from a consideration of several embodiments of my invention. For this purpose I have shown a few forms in the drawings accompanying and forming part of the present specification. I 'shall now proceed to describe these forms in detail, which illustrate the general principles of my invention; but it is to be understood'that this detailed description is not to be microphone;

Fig. 2 is a sectional view of a microphone embodying my invention, only the essential elements of which are shown; c

Fig. 3 is an enlarged plan view of the microphone shown in Fig. 2 with the diaphragm removed showing the means of subdividing the microphone into a plurality of mutually independent cells;

Fig. 4 is a view similar to Fig. 2 but of a modified form of the invention;

Fig. 5 is a sectional View of a mold or tool for constructing the invention shown in Fig. 4;

Fig.'6 is an exploded view of a mold for making that form of the invention shown in Fig. 2;

Fig. 7 is a sectional view of another modied form of my invention;

Fig. 8 is a perspective view of an assemblage of elements embodying my invention showing one manner of use; Y

Fig. 9 is a modication thereof;l

Fig. 10 is a sectional view of another form of the invention shown in Fig. 2; and

Fig. 11 is a sectional View of a further modcation thereof.

A condenser microphone l0, While will hereinafter be described in greater detail, is shown in Figure 1 as being included in a high frequency circuit. 'I'his circuit comprises a high frequency' oscillator 11 which is acted upon by the sound waves in the air in such a manner that modulated high frequency energy is supplied to primary coil 12 of the transformer 13. This modulated high frequency energy is induced in the secondary coil 14, and is applied to a variable lcondenser 15 and the condenser microphone 10 through the medium secondary coil terminals 16, 17, respectively. 'I'he condenser microphone 10 is shown as being in series with the variable condenser 15.

The secondary coil 14 is shown as being grounded at its midpoint 18. The midpoint 19 between the variable condenser 15 and the condenser microphone 10 is connected to the grid 20 of an amplifying tube 21. The plate circuit of tube 21 supplies an output to a transformer 22, and in this manner affects a load such as the ear phones 23. Variations in the capacity of the condenser microphone 10 set up by the pressure waves in the air, will alter the amount of the high frequency passed on by the amplifying tube 21 to the load 23. This is due to the fact, that the condenser microphone 10 forms a part of the high frequency circuit.

'I'he condenser microphone 10 is shown in greater detail in Fig. 2. It comprises a relatively heavy metallic -plate 24 having a smooth face to which a thin layer of insulating material 25 is applied with uniform thickness. This insulating layer 25 may be formed of a thin sheet of mica cemented to the smooth face of the metal plate 24, or a layer of phenolic condensation product, such as bakelite lacquer may be formed thereon as by coating.

Ridges of cement 26 are formed on the insulating layer 25 by printing thereon, a definite pattern such as shown in Fig. 3, using cement instead of ink. A thin metal diaphragm 27 formed of some light material such as aluminum foil or its alloys is secured to the ridges 26 of cement. In this manner, the diaphragm 27 is uniformly spacedly maintained away from the insulating layer 25 by the ridges of cement 26 and forms therewith a plurality of uniformly large air chambers 28.

I'hese air chambers 28 are formed of the same size by printing the cement ridges 26 on the insulating layer 25 in a definite pattern. This is clearly shown in Fig. 3 where the lines of cement 26 dene a series of contiguous conned hexagonal areas 29. Although the lines of cement 26 form hexagonal areas 29 as shown in Fig. 3, it is to be understood that this invention is not to be limited to the particular pattern shown, as it is understood that any pattern may be printed on the insulating layer 25 by the cement 26. It being merely essential to provide a diaphragm which is properly insulated from a back lmetallic plate, and which is maintained at a uniform distance from the insulation layer. v

The cement is printed on the insulation layer in the manner described in order to provide a series of cells such as the air chambers 28. 'Ihat portion of the diaphragm 27 above these cells ,coacts with the plate 24 to form, in effect, a plurality of mutually independent microphones. In this manner, an exceedingly large diaphragm may be employed which is capable of absorbing a large amount of sound energy from the air without' difculty in maintaining the spacing. This is not possible in the diaphragms of the prior art where the diaphragms are spacedly secured to a back plate only'along the edges. Moreover, the vibratory character of the indiivdual diaphragms, which is dened by that portion of the diaphragm immediately above the hexagonal areas 29, may be easilymade of a type which will vnot appreciably alter or distort the type or quality of sound transmitted thereby. In fact, should any one `of these individual microphones become dis-f rupted from any cause, the rest of them would function properly without impedance from the disrupted diaphragm of the particular microphone.

In Fig. 4 is shown a modified means for forming a plurality of air chambers or cells 30. The portion of the diaphragm 31 immediately above the air cell coacts with the back plate 32 to form a plurality of mutually independent microphone structures, in a manner similar to Fig. 2. An insulation layer 33 is interposed between the diaphragm 31 and the back plate 32. The elements constituting this form of the invention are similar to those hereinabove set out with respect to Fig. 2. The air cells 30 are formed by providing depressions 34 in the diaphragm 31 to form a denite pattern, such as hexagonal areas 29. This is accomplished in a manner now to be described. 'I'he metal plate 32 is first coated with an insulating layer 33. The surface of this insulating layer is then covered with a thin coating of cement and solvent which is permitted to dry. This cement is preferably composed of a material capable of being melted by heat. Any heat plastic cement such as wax, paraffin, etc. may be used.

A metallic mold 35 shown in Fig. 5 is provided with ridges 36 which form a definite pattern such as the hexagonal areas 29. This plate is heated to a point sufficient to melt the cement on the insulating layer 33 when brought in contact therewith, and a thin layer of metal foil such as the diaphragm 31 is laid upon the ridges 36 of the plate 35. The plate and foil are now abruptly brought in contact with' the insulation layer 33 and pressed firmly against the same. This causes the diaphragm 31 to become attached to the insulating layer 33 by the softening of the thermoplastic cement. It is readily apparent that due to the projections or ridges 36, the depressions 34 are formed in the diaphragm 31, and the diaphragm is attached to the insulating layer 33 only along the points of contact 37.

IFig. 6 discloses the method of forming the microphone structure shown in Fig. 2. The insulating layer 25 and the lines of cement ,26 are assembled with the metallic backing 24 vin a manner hereinabove described. The diaphragm 27 is secured to the ridges of cement 26 by means of a pair of heated pressure plates 38 and 39. The coacting surfaces 40 and 41 of the plates 38 and 39 are formed in such a manner as to smooth out all wrinkles in the metal foil diaphragm 27. The surface 40 is convex and the surface 41 concave.

In operation, the metal plate 38 is rst heated toa point suicient to melt the cement 26 when brought in contact therewith. The foil 27 is now laid upon the surface 40 while the metal layer 24 is laid across the concave surface 41 of the plate 39. 'Ihe plates 38 and 39 are now pressed rmly together so that the convex and concave surface coact to bend the foil 27 and metal plate 24 into` a slightly concave form. The degree of concavity is very slight, and insufficient to permanently deform the metal plate 24. When the pressure on the plates 38, 39 is released, plate 24 immediately springs back to its original planar form. The pressure is released only when the plates 38, 39 have cooled sufliciently to permit the cement 26 to set again.

By the method hereinabove described, it is possible to maintain the foil diaphragm 27 in an extremely flat condition. If foil of high tensile strength is employed, the diaphragm Will then be maintained in a tense position. If the foil used is soft, theV result of the treatment hereinabove described is principally to remove the wrinkles and flatten the foil.

As an alternate method to that hereinafter described, it is possble, of course, to form the device in an absolutely flat plateand thereafter to mount it in such a manner that it is bent in a direction which -leaves the foil covered face convex.

In Fig. 7, is disclosedanother modification of the microphone structure. -In this instance, air cells 42 and 43 are formed on both sides of a metal plate 44. Insulating layers 45 and 46 properly insulate the metal foil diaphragms 47 and 48 respectively from the metal plate-44. Cement vridges 49 formed in a definite pattern, assist in forming the air cells 42 and 43 and maintain the diaphragms 47 and 48 spaced from the insulation layers. 'I'his form of the invention and the method of making the same is the same as in Fig. 2.

Figure 8 discloses an assemblage of microphones which can be constructed in a manner similar to any of the methods heretofore described. Each microphone 50 of the assemblage comprises a backmetal plate 51, an insulating layer 52 and air cells 53 formed by the metal foil diaphragm 54 and the lines of cement 55.

The microphones 50 are maintained in a parallel equi-distant spaced apart relationship by means of tie-rods 56, 57, 58 and 59 passing through the corners of these microphones. These microphones may be formed in any desired width, although preferably they are formed of a width so that only one air cell 53 is formed by the coaction of the diaphragm 54 with the insulation Figure 9 discloses a further form which the microphone embodying my inventioncan assume. In this figure, the microphone assumes the form of a helix 60, the successive convolutions of which are maintained in a spaced apart relationshp by the radial tie-rods 61 and 62. A plurality of contiguous air chambers or cells 63 are formed along the length of the microphone in a manner hereinabove described. These cells are formed prior to the coiling of the microphone into the form of the helix 60. In this manner, lthe action of rolling the microphone, stretches the metal foil diaphragm 64 to remove wrinkles.

The device of Fig. 9 functions in a manner similar to the device shown in Fig. 8, in that the sound waves pass through the grating formed' by the successive convolutions of the helix. The pressure of the sound waves actingon the dia- "phragm causes the electrostatic capacity of the ar cells 63 to vary.

Fig. 10 illustrates a further modification ofthe microphone shown in Fig. 2. The operation of this form and the constituent elements thereof are essentially as in Fig. 2, but in this instance, the air cells 65 are formed by means of the raised portions 66 of the plate 67 denng a pattern such .as shown in Fig. 3. The front face of the metal plate 67 including the raised portions 66, is coated with or has cemented thereto an insulation layer 68. Cement is applied only to the raised portions 70 and the metal foil diaphragm 69 is then secured thereto as by heat and pressure. Y

In the modified form of the invention disclosed in Fig. 1l, the back metalv plate 7l is formed with a front face, having a series of cupped-out portions which are covered with an insulation layer 72 as before. The metal foil diaphragm 73 is secured to the rim portions 74 of the cups as by lines of adhesive 75. The 'foil 73, after attachment to the rims 74, is then forced inwards by thel application of extreme pressure from gas or liquid, until cup- 'shaped diaphragms are formed. In this manner,

the air cells 76 areformed. The diaphragms have the advantage of maintaining an extremely high vibratory frequency Without the necessity for tension.

The mode of operation of all the microphones hereinabove described is the same. Pressure waves in the air, striking against lsuch a diaphragm surface, will obviously force the foil towards the insulating layer of each of the cellular areas defined by the'lines of adhesives or raised portions. Due to the intervention of this insulating layer between the metal back plate and the diaphragm, all possibility of short-circuiting is obviated. Furthermore, since the insulation layer is formed of a material having a high dielectric constant, its presence does not materially affect the eiciency of the device, as the major alterations in capacity are due to alterations in the space between the foil diaphragm and theA thin insulation layer covering said face, and a diaphragm spacedly secured to the insulation rlayer by intersecting lines of cement.

2. In a microphone structure, a relatively rigid metallic backing member having a surface adapted to be directed toward a sound source, a thin,I

insulation layer covering said face, and a diaf phragm spacedly secured to the insulation layer .dene a denite pattern, aA layer of insulation material ysecured to saidr face and closely following the contour thereof, to form a composite structure, and a diaphragm'securedwith cement to the raised yportions only of the composite structure.

4. In a microphone structure, a relatively rigid rmetallic rmember ,havingy a plurality of cup-like depressions in one face thereof, a layer of insula-y tion material secured to said face and closely following the rcontour thereof to form a compositel structure, and a diaphragm uniformly vspacedr from said insulation layer and adhesively secured vthereto at spaced intervals 5. In a microphone structure, a relatively rigid metallicA member having a plurality of cup-like depressions inone face thereof, said depressions being-of the same size and closely adjacent to each other to deiine a series of rim portions, a layer of insulation material secured 4to said face and closely following the contour thereof to forma composite structure, and a diaphragm uniformly spaced from said insulation layer and adhesively secured thereto only at the yrim portion.

6. The method of constructing a microphone of the type described, comprising: forming .a layer of insulation material on a metal backing, coating the insulation material with a' thermoplastic cement and permitting it to dry, laying a metal foil diaphragm on the cement, and applying heat to the diaphragm along intersecting lines forming a definite. pattern and to a point sulficient to melt the cement underlying the pattern lines.

l'1. The method of constructing a. microphone of the type described, comprising: forming a ,l layer of insulation material on a metal backing, f f coating the insulation material with a thermoplastic cementy to form a cement pattern thereon 95 and permitting it to dry, bending the compositey v material with the cement on the concave side, laying a metal foil diaphragm on the concave side under pressure, applying heat to the diaphragmy suiiicientl to melt the underlying cement pattern, yandfreleasing the pressure after cooling of the cement. Y

8. The method of constructing a sound device, which comprises overlying a metal plate with anA adhesive in thin ridges to form a series ofy de- 105 pressedl cellularl areas, ilexing said plate, so that the cellular side is concave, attaching a thin dia phragm to the ridges so that yitv overliesfail of the areas, and then reducing the concavity so as to stretch the diaphragm. f

FRANK REBER.

11ul f 

